1. Field
The present invention relates generally to communication, and more specifically to techniques for supporting GSM to W-CDMA reselection.
2. Background
Wireless communication systems are widely deployed to provide various types of services such as voice, packet data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources. Examples of such multiple-access systems include time division multiple access (TDMA) systems and code division multiple access (CDMA) systems. A TDMA system may implement one or more standards such as Global System for Mobile Communications (GSM). A CDMA system may implement one or more standards such as Wideband CDMA (W-CDMA), IS-2000, and IS-95. GSM and IS-95 are second generation (2G) standards for TDMA and CDMA, respectively, and W-CDMA and IS-2000 are both third generation (3G) standards for CDMA. These standards are well known in the art.
GSM is a radio access technology (RAT) that can provide voice service and low to medium rate packet data service. GSM networks are widely deployed throughout the world. W-CDMA is a new radio access technology that can provide enhanced services and capabilities (e.g., higher data rates, concurrent voice and data calls, and so on). One of the key requirements in the development of W-CDMA is to enable seamless operation with existing GSM networks. W-CDMA thus provides various capabilities to enable interworking with GSM networks. By providing these capabilities, multi-RAT terminals (e.g., dual-mode cellular phones) that support both W-CDMA and GSM may be manufactured and used by subscribers to obtain the performance advantages of W-CDMA and the coverage benefits of GSM, all with the same terminal.
To support seamless operation with W-CDMA networks, additional capabilities are defined for GSM and specified in Release 99 version of the GSM standard. One of these additional capabilities is the ability for a GSM network to broadcast information for neighboring W-CDMA cells. This broadcast information allows dual-mode terminals operating on the GSM network to learn of the presence of W-CDMA cells. Moreover, the broadcast information includes cell-specific information that may be used by the dual-mode terminals to quickly acquire the W-CDMA cells. Such cell-specific information includes, for each W-CDMA cell, (1) the frequency and primary scrambling code used by the W-CDMA cell and (2) whether or not diversity mode is employed by the W-CDMA cell.
W-CDMA and GSM each provide a “cell reselection” process whereby a terminal operating on one network determines a suitable cell in another network from which it plans to receive available services. A “cell” can refer to a base station in a network and/or the coverage area of the base station, depending on the context in which the term is used. Cell reselection procedures are performed while the terminal is in idle mode so that even as the environment of the terminal changes (e.g., due to movement of the terminal or changing channel conditions), the terminal can monitor the network for incoming pages and be prepared to originate a connection whenever the user desires.
A dual-mode terminal can measure both GSM neighbor cells and W-CDMA neighbor cells to determine the best cell on which to “camp”. If the dual-mode terminal is currently camped on a GSM cell, then the measurements for W-CDMA neighbor cells may be made based on the cell-specific information broadcast by the GSM cell. If the measurements and the cell reselection procedures indicate that there is a W-CDMA cell ranked higher than the current GSM cell, then the terminal can switch to W-CDMA mode and camp on the better W-CDMA cell.
Currently, most deployed GSM networks support either Release 97 or Release 98 version of the GSM standard. These GSM networks are not able to broadcast information for neighboring W-CDMA cells in the manner specified in the Release 99 version of the GSM standard. A dual-mode terminal operating in a “pre-Release 99” GSM network (i.e., one supporting a release prior to Release 99) may not be aware of neighboring W-CDMA cells and may therefore be “stuck” on the GSM network.
There is therefore a need in the art for techniques to support cell reselection from GSM to W-CDMA for pre-Release 99 GSM networks.